>>26105
The great majority of people use just photoshop, GIMP, or SAI for texture modding. For baking or if you want to get super fancy, you can use 3d programs, and/or substance painter.
Substance painter is great, but it's NOT a substitute for actually learning how to shade, color, add texture, paint details, etc. You can apply pretty much anything you learn from digital painting tutorials to texturing. When you're ready to move to the next step and paint directly onto models, you can use 3dcoat, substance painter, or even just zbrush or blender. It depends on what you want. I seriously recommend actually learning to paint them a bit first, because a lot of beginners I see who texture straight from 3d programs end up making really flat and dead-looking textures, while not knowing what makes a texture look good in the first place.
When texturing, keep in mind that you want your texture to look good on low and high settings. Don't go overboard painting in details that aren't modeled in; it doesn't look good at high settings, where shadows won't look right in those areas. At the same time, do not leave areas looking flat; they should have some soft gradation at the very least. You can't rely on other people to have shadows on, so you do have to give them some shading.
If you have AO bakes (ambient occlusion), use them. These are shadows that are cast when all sides of an object are brightly lit. AO bakes shade in crevices where light wouldn't go on your mesh in the first place, so they're a great starting point.
If you have other bakes available, use them too! I combine bakes so that an object looks okay from most angles.
I honestly highly doubt you want to make the kind of appliers described above. If you mean appliers for bodies people are already using, start with Omega appliers. The dev kit is free:
https://marketplace.secondlife.com/p/Omega-System-Dev-Kit/6070969
You upload textures, right click them in your inventory, copy UUID, open up the "config" script in the Omega applier, and paste the UUIDs in. Its very simple once you get the hang of it.